The broad objective of the proposed research is to determine how closely the integrated surface EMG reflects the total muscle fiber activity occurring during different types of human voluntary contractions, in various muscles with different anatomical and physical properties. The EMG/Force relationship will be studied for each muscle (or group of muscles) during isometric contractions and when shortening or being stretched at a number of controlled velocities. The activity in synergists and antagonists will also be investigated. The results will be analyzed to determine whether integrated surface EMG can be used as a means of measuring the force/velocity relationship of intact human muscles and how this may change with the intensity of the contraction in heterogeneous muscles if larger, faster fibers are recruited sequentially.